I noticed the dinette seat seems to be uneven (lower on the side I am regularly seating on.) I tried reversing the plywood board, but that didn't help, but reversing the seat cushion did. It seems the cushion got crashed thinner by my weight. And that is on a 2 month old trailer.

I wonder if it is possible to replace the foam insert with something that will last.

Rotate the cushions errr... where you sit on them and crush them ALL out. Hopefully they all find a common place. Winterization time this coming fall take it in for your dealer to do warranty work with this on your list if they do not crush out evenly.

Try rotating your cushions every 2-3 weeks. Well, if you're using the couch daily.
I would also suggest rotating your mattress every 2 months.
Maybe that sounds excessive, I dunno, that's what I do with my living room couch cushions though